The monoclonal antibody Lan3-1 binds to a small set of different neurons in the leech central nervous system. Two different Lan3-1 labeled neurons are already identified. They communicate synaptically and constitute a subnetwork within the greater reproductive circuits. We propose to identify the remaining Lan3-1 labeled neurons to determine (1) whether the Lan3-1 antigen specifies synaptically linked neurons or (2) whether the antigen also occurs in neurons that are integrated into different networks. In the latter case, we propose to analyze the detailed physiological behavior of one or two Lan3-1-labeled neurons subserving behaviors other than sex to screen for another common functional denominator. Thus, we propose an electrophysiological analysis of a monoclonal antibody staining pattern in an attempt to trace the functional significance of such an example of molecular neuroanatomy. In addition, we will carry out an interspecies comparison of Lan-3-1 staining patterns in two closely related leech species, Haemopis marmorata, against which the antibody was generated and Hirudo medicinalis with which Lan3-1 corossreacts but with a simpler staining pattern. Our immunological probe and electrophysiological data promise such an interspecies comparison to contribute to our understanding of network evolution at the cellular level.